1. Field of the Invention
The present invention relates to the improvements of an air/fuel ratio control system of an internal combustion engine with an emission control system having at least a catalyst, and specifically to an air/fuel ratio control system capable of controlling an air/fuel mixture ratio so that the quantity of oxygen stored in the catalyst is adjusted toward a desired quantity.
2. Description of the Prior Art
In recent years, an automotive vehicle often uses a three-way catalyst to reduce oxides of nitrogen (BOX), unburned hydrocarbons (HCl), and carbon monoxide (CO). On automotive vehicles employing a three-way catalytic converter (a three-way catalyst) in the exhaust passage, in order to detect an air/fuel mixture ratio (often abbreviated to "A/F ratio"), an A/F ratio sensor, such as an O.sub.2 sensor, is usually provided in the exhaust passage upstream of the three-way catalyst. As is generally known, the purpose of the A/F sensor such as oxygen sensor is to monitor the percentage of oxygen contained within the exhaust gases at all times when the engine is running, so that the EMU can maintain the A/F ratio at as close to stoichiometric as possible. A voltage signal from the A/F ratio sensor varies depending on the air/fuel mixture ratio. In automotive vehicles with both a three-way catalyst and an A/F ratio sensor located upstream of the three-way catalyst, an electronic engine control unit (ECU) or an electronic engine control module (ECM) generally utilizes the deviation of an A/F ratio sensed by the A/F ratio sensor from a stoichiometric air/fuel ratio to arithmetically calculate or estimate the quantity of oxygen stored in the three-way catalyst. The ECU controls the A/F ratio such that the estimate (the calculated value) of the quantity of air (oxygen) stored in the catalyst is adjusted to a desired value (for example, one-half of a limit value of the quantity of oxygen stored in the three-way catalyst). When the A/F ratio is lean (excess air), air (oxygen) is adsorbed or trapped by the three-way catalyst and stored in the catalyst. Conversely, when the A/F ratio is rich (too much fuel), air (oxygen) is desorbed or released from the three-way catalyst. Generally, an oxygen desorption speed at which oxygen is desorbed from the three-way catalyst is lower than an oxygen adsorption speed at which oxygen is adsorbed by the three-way catalyst. For the reasons discussed above, the ECU increasingly compensates for the quantity of oxygen stored in the three-way catalyst, which quantity will be hereinafter referred to as an "oxygen storage quantity", by increasing an increment for a calculated value (or an estimate) of oxygen storage quantity, when the sensed A/F ratio is lean (excess air). To the contrary, when the sensed A/F ratio is rich (too much fuel), the ECU decreasingly compensates for the oxygen storage quantity, by decreasing a decrement for the calculated value (or the estimate) of oxygen storage quantity. Such A/F ratio control systems have been disclosed in Japanese Patent Provisional Publication Nos. 9-310635 and 6-249028.